1. Field of the Invention
The present invention relates to a nitride semiconductor light emitting device and a method for manufacturing the same, and more particularly, a nitride semiconductor light emitting device which improves light extraction efficiency and a method for manufacturing the same.
2. Discussion of the Related Art
In general, a nitride Light Emitting Diode (LED) has a light emitting region including ultraviolet ray, blue light, and green light. Particularly, a GaN-based nitride semiconductor LED is applied to optical devices of blue/green LEDs, and high-switching and high-output electronic devices, such as Metal Semiconductor Field Effect Transistors (MESFET) and Hetero junction Field-Effect Transistors (HEMT).
FIG. 1 is a longitudinal-sectional view illustrating a conventional nitride semiconductor light emitting device.
As shown in FIG. 1, the conventional nitride semiconductor light emitting device includes a buffer layer 12 on a substrate 11, an n-GaN layer 13 on the buffer layer 12, an active layer 14 formed in a multiple quantum well structure to emit light, a p-GaN layer 15, and a transparent electrode 16.
Here, after the n-GaN layer 13 is exposed to the outside by selectively etching the transparent electrode 16 to the n-GaN layer 13, an n-type electrode 18 is formed on the exposed n-GaN layer 13, and a p-type electrode 17 is formed on the transparent electrode 16.
The above conventional nitride semiconductor light emitting device has a principle that photons are generated by electron-hole recombination at the active layer 14 between a P/N junction and the photons escape to the outside of the light emitting device, thereby generating light.
FIG. 2A is a longitudinal-sectional view schematically illustrating a final shape of the substrate in the conventional nitride semiconductor light emitting device, and FIG. 2B is a view illustrating extraction of light generated from the inside of the substrate of FIG. 2A to the outside.
In order to increase light extraction efficiency of the conventional nitride semiconductor light emitting device, a nitride semiconductor growth technique or a method of changing the structure of the device using a chip process is used. Otherwise, the light extraction efficiency may be increased through surface treatment of a certain material.
First, if roughness of a p-GaN layer is achieved according to nitride semiconductor growth requirements, internal quantum efficiency is lowered due to diffusion of magnesium (Mg). However, external quantum efficiency may be improved. Further, in order to use roughness of the p-GaN layer, a technical drawback, such as elongation of a cycle of multiple quantum wells, may be encountered.
Further, the conventional p-GaN layer having a thickness of 150˜200 nm cannot be uniformly patterned, and thus the thickness of the p-GaN layer needs to be artificially increased. However, if the p-GaN layer has an increased thickness through growth, resistance of p-GaN layer is increased, and current crowding occurs due to the fact that dispersion of current is carried out in the vertical direction rather than in the horizontal direction, thereby causing an increase in operating voltage.
Although the p-GaN layer having an increased thickness is obtained through growth, the p-GaN layer must have a high-quality surface. However, in order to prevent degradation of the active layer during growing of the p-GaN layer, the p-GaN layer is grown at a low temperature. If the thickness of the p-GaN layer grown at a low temperature is increased, then the p-GaN layer cannot have a high-quality surface and thus the performance of the device is lowered. A doping concentration of magnesium (Mg) used to form the p-GaN layer and growth conditions of the P-GaN layer are factors greatly influencing the shape of the surface of the p-GaN layer, and thus high precision is required when performing a growth process.
Second, regarding to changing the structure of a device using the chip process, a part of a sapphire substrate or LED GaN layer is formed to a reverse mesa structure so as to prevent generated light from being confined therein and lost, as a chip shaping technique. In order to form the part of the sapphire substrate or LED GaN layer to a reverse mesa structure, wet etching is mainly used and an etching solution of the wet etching is one out of strong acids, such as sulfuric acid, phosphoric acid, and nitric acid, a base, such as potassium hydroxide, or mixtures of at least two thereof. In order to perform wet etching of a nitride semiconductor compound, a high temperature is required. In terms of properties of wet etching, it is difficult to achieve precise control and concentration set, and use of a mixture of phosphoric acid and sulfuric acid, which is mainly used, is dangerous.
Finally, a method of increasing light extraction efficiency of a nitride semiconductor light emitting device by roughing a transparent conductive film, such as ITO, is used. The transparent conductive film is roughed by changing parameters of a deposition process, or wet etching is used. However, this method has a drawback, such as difficulty in verifying process reproducibility.